I've been using K9 as well and I'm not thrilled with it either. I was using Seven as a client on Symbian til I switched to Android and apparently they've got a beta program for Android as well. The app itself is free, the only charge is for first use and setting up each account in form of an international text to the Isle of Man (I know how shady that sounds but I've had my account with them for at least a year and no other charges ever appeared). Here's where to download it: http://community.seven.com/download.php?target=ce_disclaim

I only tried it for a few minutes, but it worked seamlessly with my Yahoo account and has support for aol and hotmail, however, exchange server email seems to fail for my employer so I'll have to keep looking. If all you need is aol and hotmail, I think you'll find it works quite well and it's got a nice interface and widget that comes with it.

Personally, the only reason I installed K9 was for the email widget, but it's much inferior to the Moto Blur one that came with my Droid X. Other than the widget it doesn't really do much more than the stock client for me.